1. Field of the Invention
The present invention relates to a method and apparatus for providing anodic protection for submerged metal structures on marine vessels.
2. Description of the Prior Art
It is a well known fact that submerged metallic structures on marine vessels are susceptible to attack and disintegration by salt water. The hulls of most pleasure marine craft which are currently being manufactured are constructed of fiberglass embedded in polyester or epoxy resin. In earlier years the hulls of marine vessels were typically constructed of wood. While neither fiberglass nor wood are electrolytically attacked by salt water, virtually all marine vessels require various fittings and drive components which must be formed of metal for strength and which are always submerged when the vessel is in the water. For example, any boat having an inboard engine must necessarily have a propeller shaft extending through a sealed drive shaft opening and a propeller mounted upon the shaft. Also, the propeller shaft must be stabilized by a metal propeller shaft supporting strut. In addition to drive components, virtually all marine vessels have numerous through-holes for cooling water intakes and exhausts, bilge pump outlets, flushing water intakes, and the like. All such through-holes are conventionally lined with annular metal fittings to provide rigidly at points which would otherwise be structurally weak openings in the hull.
All metal fittings and drive components which are maintained in a submerged condition on the underside of a hull of a marine vessel are formed of those metals which are the least susceptible to the strong electrolytic action created by the presence of brine in seawater. Such fittings and drive components are typically constructed of brass, bronze, or silicone bronze. However, although electrolytic destruction of these metals is quite slow, even these metals will deteriorate and disintegrate when maintained in a submerged condition over a prolonged period of time.
To prevent the destruction of vital hull fittings and drive components, sacrificial anodes have long been employed in the vicinity of the metal fittings and drive components on the undersides of the hulls of marine vessels. Sacrificial anodes are formed of metals which have far greater susceptibility to electrolytic action than do the metals forming the hull fittings and drive components. Consequently, electrolytic attack at the underside of the hull operates primarily against the sacrificial anode, rather than the hull fittings and drive components. When the sacrificial anode has been largely destroyed by electrolytic attack, it is merely replaced. The structural integrity of the metal hull fittings and drive components is thereby preserved.
Conventional sacrificial anodes designed for use of marine vessels typically are constructed as slabs or plates, usually about one half inch in thickness, about six inches in width and about twelve inches in length. In the initial fabrication of marine vessels at least one pair of parallel, threaded studs are normally embedded in the hull to project externally therefrom below the waterline of the vessel. The studs are used to mount the slab-like or plate-like sacrificial anodes.
In conventional practice, sacrificial anodes are normally replaced by a diver operating submerged below the surface of the water. Replacement is typically performed as an incidental service in conjunction with cleaning marine growth from the underside of the hull. The hulls of most marine vessels need to be cleaned of marine growth, typically every two to four weeks. The diver who cleans the hull of a marine vessel normally checks the condition of the sacrificial anode at the time the hull cleaning operation is performed. If the anode has suffered significant electrolytic attack, it must be replaced.
When a diver recognizes that an anode requires replacement, the conventional procedure is to first measure the diameter and spacing of the studs adapted to hold the sacrificial anode in position. The diver thereby creates a pattern while beneath the surface of the water on a first occasion. After leaving the water on that first occasion the diver takes the pattern to his workshop and drills holes in a replacement sacrificial anode of a size and spacing according to the pattern. While most pleasure craft currently being manufactured employ anode mounting studs which are three-eights of an inch in diameter, the spacing between these studs varies considerably between boat manufacturers, models of boats constructed by the same manufacturer, and even among different boats of the same model.
When anode mounting studs are embedded in the hull of the craft during fabrication of the hull, they are normally spaced in pairs, with the spacing of the studs within each pair varying from between about two to about six inches, center-to-center. However, because there is such a wide variance in center-to-center spacing among individual vessels within this range, it is impractical for sacrificial anodes to be predrilled and installed while cleaning the hull on the first occasion of entry into the water by the diver. This would require stocking an excessive number of sacrificial anodes having a wide variety of different mounting aperture spacings. Moreover, the diver does not know the size and spacing of the anode mounting studs until he can actually take these measurements on the first occasion of entry into the water. Consequently, the replacement of a sacrificial anode currently requires two dives for each replacement effort, and drilling of holes in a new sacrificial anode between these dives.
The present practice in replacing sacrificial anodes is quite time consuming and wasteful. A great deal of the time of replacement is expended by a diver in suiting up and reentering the water in the vicinity of a vessel which has already had its hull cleaned. Also, the diver must expend further time in traveling to the vessel for the second dive.